1. Field of the Invention
The present invention relates to a wiring board, and an electronic device including the wiring board as well as a method of mounting electronic parts on the wiring board, and more particularly to a wiring board suitable for mounting an electronic part thereon by use of a lead-less solder and an electronic device including a wiring board, on which electronic parts are mounted via the lead-less solder, as well as a method of mounting electronic parts on a wiring board by use of the lead-less solder.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
FIG. 1 is a fragmentary cross sectional elevation view of a first conventional structure of a wiring board with through holes for mounting an electronic part thereon. FIG. 2 is a fragmentary cross sectional elevation view of the first conventional structure of the wiring board, on which the electronic part is mounted via the through holes. The first conventional structure of the wiring board will be described.
A body of a wiring board 110A comprises a copper-clad lamination substrate 11. This copper-clad lamination substrate 11 comprises an insulating sheet having surfaces coated with copper foils. The insulating sheet may comprise an insulating base material, into which a resin material is infiltrated. Typical examples of the insulating base material may include, but not limited to, paper base materials, glass base materials and a polyester fiber base material. Typical examples of the resin material, to be infiltrated into the insulating base material, may include, but not limited to, an epoxy resin and a phenol resin.
The copper-clad lamination substrate 11 includes at lest a through hole 12. A typical example of shape of the through hole 12 may include, but not limited to, a cylinder shape. An inner wall of the through hole 12 is coated with an electrically conductive film 13 which is further connected or communicated with the copper foils on the surfaces of the copper-clad lamination substrate 11. This electrically conductive film 13 may be formed as follows. A catalyst is applied onto the inner wall of the through hole 12, before an electroless-copper-plating is taken place to form a base copper-plated layer on the inner wall of the through hole 12. Subsequently, an electro-copper-plating is taken place to form a copper-plated layer on the base copper-plated layer, thereby to form the electrically conductive film 13 on the inner wall of the through hole 12. The through hole 12 with the inner wall coated with the electrically conductive film 13 will, hereinafter, be referred to as a through hole 14.
Each of the copper foils on the opposite surfaces of the copper-clad lamination substrate 11 is selectively removed or etched, so that the remaining copper foil on each of the surfaces of the copper-clad lamination substrate 11 comprises a land 15 extending around the through hole 14 and circuit wirings 16 which are connected with the land 15. A typical plan shape of the land 15 is a ring shape, provided that the through hole 14 has a cylinder shape. Typically, the land 15 and the circuit wirings 16 are formed on the opposite surfaces of the copper-clad lamination substrate 11. It may optionally be possible that the land 15 and the circuit wirings 16 are formed on only one surface of the copper-clad lamination substrate 11. The land 15 is preferably small as realizing a high density packaging as possible, as long as a minimum necessary bonding strength is ensured.
The opposite surfaces of the copper-clad lamination substrate 11 are covered by solder resist layers 17, except for the lands 15 and peripheral portions thereof. For example, the circuit wirings 16 on each of the opposite surfaces of the copper-clad lamination substrate 11 are covered by the solder resist layer 17. The solder resist layer 17 serves as a protection layer which protects the surface of the copper-clad lamination substrate 11 from soldering with a tin-lead solder 31, except for the land 15 which is soldered with the tin-lead solder 31. The solder resist layer 17 may be formed by printing a paste on the each surface of the copper-clad lamination substrate 11 and subsequent exposure to a light. The solder resist layer 17 is so formed as not covering the land 15, in order to allow formation of a fillet 31A of the tin-lead solder 31 without any disturbance.
An electric part 20 is mounted on the wiring board 10A. The electric part 20 has a body 21 and a plurality of leads 22, one of which is shown in FIG. 2. The lead 22 is inserted into the through hole 14 of the wiring board 110A, so that the lead 22 completely penetrates the through hole 14, whereby a top of the lead 22 projects from the opposite surface to the surface in the side of the electric part 20. The lead 22 is bonded to the through hole 14 via the tin-lead solder 31. Since the solder resist layer 17 is so formed as not covering the land 15, the fillet 31A of the tin-lead solder 31 is formed. A typical example of the tin-lead solder 31 is a tin-lead eutectic solder containing 63% by weight of Sn and 37% by weight of Pb, which will hereinafter be referred to as Pb-63Sn. The tin-lead solder 31 relaxes a stress which is caused by a miss-match or a difference in thermal expansion coefficient between different materials of the lead 22 and the copper-clad lamination substrate 11, whereby no defect is caused on the connection between the electric part 20 and the wiring board 110A.
The use of the tin-lead solder 31 is not preferable in view that the lead provides an environmental impact. For this reason, some lead-less solders have been often used recently in view of the environmental requirement. A typical one of the lead-less solders includes tin as a main component, and further silver, copper, zinc, bismuth, indium, antimony, nickel, and germanium as additional components. The above-described Pb-63Sn solder has a melting point of 183° C. A melting point of the lead-less solder is ranged from 190° C.-230° C., which is higher than the melting point of the Pb-63Sn solder.
A base material of the wiring board 110A is an epoxy-based material which has a glass transition temperature in the range of 125° C.-140° C. The use of the lead-less solder instead of the above Pb-63Sn solder results in an increased difference in solidifying shrinkage temperature between the solder and the copper-clad lamination substrate 11 of the wiring board 110A. The copper-clad lamination substrate 11 shows an expansion in the soldering process, and a contraction after the soldering process. The lead-less solder shows a larger tensile strength and a larger creep strength than the above Pb-63Sn solder. The lead-less solder shows a smaller elongation as compared to the above Pb-63Sn solder. Those properties of the lead-less solder disturb the desired stress relaxation. The use of the lead-less solder to mount the electric part 20 on the conventional wiring board 110A often causes a peel of the land 15, even the use of the above Pb-63Sn solder does not cause any peel of the land 15.
FIG. 3 is a photograph which shows the land 15 as peeled and floated from the surface of the copper-clad lamination substrate 11 of the wiring board 110A through a soldering process using a lead-less solder 32. FIG. 4 is a photograph showing that a boundary between the land 15 and the circuit wiring 16 is deformed and disconnected after the temperature cyclic test has been carried out. If the soldering process using a lead-less solder 32 is applied to the conventional wiring board 110A, then the land 15 is peeled and floated from the surface of the copper-clad lamination substrate 11. The circuit wiring 16 connected with the land 15 is also risen upon floating of the land 15, whereby the circuit wiring 16 receives an excessive tensile stress. Thereafter, 200 cycles of heating and cooling processes are taken place as a temperature cyclic test in order to apply a thermal stress to the wiring board 110A, whereby the boundary between the land 15 and the circuit wiring 16 is largely deformed and disconnected as shown in FIG. 4. This verifying test verifies that the combined use of the lead-less solder and the conventional wiring board 110A results in a remarkable deterioration in reliability of the electronic device which includes the electronic part 20 mounted on the wiring board 110A via the lead-less solder.
Japanese laid-open patent publication No. 2001-332851 discloses a conventional method of suppressing the peel of the land. FIG. 5 is a fragmentary cross sectional elevation view of a second conventional structure of a wiring board, on which an electronic part is mounted via the through holes, as disclosed in the above Japanese publication. This second conventional structure of the wiring board shown in FIG. 5 is different from the above-described first conventional structure of the wiring board shown in FIG. 2 in that a wiring board 110B has a modified solder resist layer 117 having an extension portion 117A, which overlies a peripheral region of the land 15 for suppressing or preventing the land 15 from being peeled.
It is the fact that a large number of the read-made products are as shown in FIG. 2 and free of any countermeasure to suppress the peeling of the land 15. In case that the electronic part 20 becomes defect and should be replaced by a new electronic part, if the new electronic part may be mounted on the wiring board 110A by use of a lead-less solder 32, then this may cause that the land 15 is peeled as described above. In order to avoid this problem, it may be effective that instead of the above wiring board 110A, the new electronic part is mounted onto the wiring board 1110B.
Manufacturing of the wiring board 110B needs to design a new jig for printing a predetermined pattern of a paste of the solder resist 117 and formation of the solder resist layer 117 by use of the newly designed jig. This may result in an increased manufacturing cost of the repaired product. Disposal of the above wiring board 110A makes the non-defective wiring board 110A wasted. These disadvantages in manufacturing of the wiring board 110B are caused not only in repairing the ready-made products but also manufacturing the new products.
In the above circumstances, the development of a novel technique free from the above problems is desirable.